Field of Applicable Technology
The present invention relates to a flat configuration image display apparatus, and in particular to a display apparatus based on a flat configuration color CRT (cathode ray tube) for use in equipment such as color television receivers, computer display terminal, etc.
In the prior art, several types of display apparatus based on a flat configuration CRT have been described, for example in Japanese Patent Laid-open No. 54-143063 and 55-33734. With such an image display apparatus, a plurality of thin line electron beams are derived from one or more respective flat-shaped electron beams produced from an electron source which includes at least one line cathode. A periodically varying voltage is applied between vertical deflection electrodes through which the line beams are passed, to execute vertical scanning, and thereafter each electron beam is deflected horizontally by horizontal deflection electrodes to which a horizontal scanning voltage is applied. Each electron beam then falls on a pattern of fluorescent layer portions which are formed on a flat transparent substrate, these fluorescent layer portions generally consisting of a parallel array of vertically aligned stripes formed as a repeated pattern of red-emission, green-emission and blue-emission fluorescent stripes. Images, characters, etc. are thereby displayed, by appropriately modulating each electron beam.
A typical configuration for such a prior art flat configuration image display apparatus will be described in the following, referring to FIG. 1. In FIG. 1, numeral 1 denotes one of a set of parallel horizontally extending line cathodes, each of which is formed of tungsten wire having a diameter of from 10 microns to several tens of microns and having a layer of electron-emissive oxide material coated thereon to a thickness of several microns to several tens of microns. A predetermined value of voltage is applied between the ends of a line cathode to heat the cathode to a temperature of 600.degree. to 800.degree. C., whereby electrons are uniformly emitted from the emissive oxide material. With one method or controlling this cathode heating which has been proposed, as shown in FIG. 2, a DC voltage E.sub.k for producing heating is switched on and off in synchronism with the vertical synchronizing signal (1V), while a pulse voltage E.sub.kp is applied to the cathode 1 when the voltage E.sub.k is off. Each pulse of voltage E.sub.kp is applied only for the duration of an interval in which electron emission is required. Usually, a plurality of these line cathodes are used, arranged as a vertically extending array of horizontally oriented line cathodes, with rows of electron beams derived from successive ones of the line cathodes being used to scan successive sets of lines of a video signal frame. To do this, pulses of the form shown in FIG. 2 are sequentially applied to the plurality of line cathodes 1, to select one cathode at a time for emission of electrons to form a row of electron beams. Each selected row of electron beams is horizontally and vertically deflected to successively produce a set of successive scan lines of the display, i.e. each electron beam forms one block of the display picture, whereby frame of a television picture is formed each time that all of the cathodes are successively selected.
Numeral 2 denotes a rear electrode, formed of a metal plate or formed by a process such as vacuum evaporative deposition or sputtering of a conducting film such as a metallic film or a transparent conducting film on an internal face of an external enclosure (not shown in the drawing) of the display apparatus. The rear electrode 2 is held at a potential such that an electron beam which is generated by heating a line cathode 1 is impelled in a predetermined direction. Numeral 3 denotes an electron beam extraction electrode, for extracting electron beams that are generated from the line cathodes 1. The electron beam extraction electrode 3 has through-apertures 3a formed therein for extracting the electron beams 11, with these apertures being positioned opposite respective ones of the line cathodes 1. The shape, dimensions and number of these through-apertures 3a are determined by the required numbers of electron beam spots, the required level of electron beam current, etc. Numeral 4 denotes vertical deflection electrodes, consisting of conducting electrodes 4a which are formed on both sides of substrates made of an insulating material, by a process such as evaporative deposition, screen printing, etc, and which are subjected to periodically varying vertical scanning voltages to deflect the electron beams 11 in the vertical direction of the display picture. Numeral 5 denotes modulation electrodes, which are divided into vertical-extending separate electrodes for respective ones of each horizontally extending row of electron beams 11 generated from a line cathode 1. Modulation signals are applied to these modulation electrodes in synchronism with horizontal and vertical scanning, to modulate the electron beam levels in accordance with a video signal. Numeral 6 denotes a shield electrode which is held at a fixed potential and serves to mutually shield the electrodes which are positioned on either side thereof. Numeral 7 denotes horizontal deflection electrodes, which are divided into two comb-shaped parts and which are subjected to periodically varying horizontal deflection voltages for deflecting the electron beams 11 in the horizontal direction. Numeral 8 denotes accelerator electrodes which are coupled to a fixed high level of DC voltage, to accelerate the electron beam 11, and 9 denotes a transparent substrate formed of a material such as glass. In general, the faceplate of a glass outer enclosure of the CRT is used as this transparent substrate 9, with a vacuum being of course maintained within that enclosure. On the inner (vacuum) surface of the substrate 9 is formed a photoemissive section 10, consisting of a set of vertically extending stripe-shaped layer portions of fluorescent material arranged in a pattern consisting of successive repetitions of a set of red, green and blue-emission stripes, and a metal back layer, the latter being formed of aluminum film. Normally, the high DC voltage (e.g. 5 to 20 KV) that is applied to the acceleration electrodes is also applied to this metal back layer. Although it will be assumed throughout the following that the fluorescent layer portions are formed as continuous vertical stripes, other patterns based on vertical columns of fluorescent layer portions could also be envisaged.
Each of the display blocks has a fixed width (determined by the number of fluorescent layer portions of a block) and a fixed height (determined by the number of scan line portions in a block). One complete scan line of the display is formed of the respective scan line portions of a horizontally extending set of display blocks, and in this way the blocks combine to form a single image on the fluorescent layer surface. Such an apparatus has a simple configuration, a high level of brightness, high resolution, and thin shape.
However with such a prior art flat configuration image display apparatus, as shown in FIG. 3, a position deviation may occur at the time of assembly of the apparatus, whereby the stripe pattern of the fluorescent layer portions 10a is axially rotated with respect to the horizontal deflection electrodes 7. In FIG. 1, 11' and 11" respectively denote two electron beams of an uppermost and a lowermost set of vertically aligned display blocks. That is to say, the respective beam landing positions of the beams 11" and 11', in the horizontally undeflected condition, are vertically mutually aligned. If there were no position deviation such as that shown in FIG. 5, then in this horizontally undeflected condition, the uppermost beam landing position of the upper beam 11' (i.e. during scanning of the uppermost line of the display picture) and the lowermost beam landing position of the lower beam 11" (i.e. during scanning of the lowermost scan line) would coincide with a single one of the color stripes 10a, which in this example is a green-emission stripe. However if there is a component position deviation such as that shown in FIG. 5, then when periodic horizontal deflection is applied to the electron beams 11 such as to produce display of an image, there will be color errors between the upper and lower portions of the displayed image.
As a result, due to the very small dimensions which are involved, it is extremely difficult in practice to manufacture such a display apparatus with a sufficient degree of consistent positioning accuracy to ensure a high manufacturing yield.